cost estimation of using an absorption refrigeration system

GEOTHERMAL TRAINING PROGRAMME Reports 2007 Orkustofnun, Grensásvegur 9, Number 4 IS-108 Reykjavík, Iceland

25

COST ESTIMATION OF USING AN ABSORPTION REFRIGERATION SYSTEM WITH GEOTHERMAL ENERGY FOR

INDUSTRIAL APPLICATIONS IN EL SALVADOR

Juan Carlos Ábrego Castillo LaGeo S.A de C.V.

15 ave. Sur, Colonia Utila, Santa Tecla EL SALVADOR

[email protected]

ABSTRACT

El Salvador is interested in improving the efficient utilization of its available resources, especially those that are environmentally friendly. Natural geothermal resources are available and have been used for commercial electrical production since 1975. Implementation of a project of installing and operating a cooling and refrigeration facility, using geothermal fluid as the heating source in combination with an absorption cycle, is being considered. The Lithium Bromide absorption cycle is thought to be feasible for cooling with a COP (coefficient of performance) of about 0.6. A refrigeration absorption cycle with a mixture of ammonia and water cannot be easily adapted to the climatic conditions of El Salvador, as it needs low cooling temperatures for equipment, such as condenser and absorber. It is recommended that a combined application of a cooling and a refrigeration cycle is studied in more detail.

1. INTRODUCTION El Salvador is located in Central America. Its capital city, San Salvador, is located 13°50´ north of the Equator and 88°55´ west of Greenwich, facing the Pacific Ocean, as shown in Figure 1. Due to its location, the climate is tropical, with ambient temperatures ranging from 16 to 36°C in the wet and dry seasons. Electrical generation is distributed as shown in Figure 2, with generation from thermal sources being the strongest contributor at 42 %. (UT, 2007). The electrical demand over the next five years is expected to be covered by expansion of the existing types of electricity generation in addition to coal usage, shown in Figure 3 (Rodríguez and Herrera, 2007). FIGURE 1: Map of El Salvador,

Central America

Ábrego C. 26 Report 4

The National Authority has developed an agenda to make more efficient use of electricity and to generate energy savings. This commitment has resulted in the issuance of rules for the use of lighting systems, air conditioners, refrigeration and motors (Ministry of Economy, 2007). The local economy includes the production of food which also includes the cooling and refrigerating costs for storing the products meant not only for consumption within the country, but also for export to foreign markets such as USA with the TLC (TLC: Free Market Treaty between El Salvador and USA). In El Salvador, the Ahuachapán and Berlín geothermal fields are under commercial production for a total electricity generation of 167 MW which is supplied to the grid. In this report, the same geothermal energy is proposed for use in combination with refrigeration and cooling technology using an absorption cycle to offer an environmentally friendly alternative for the

storage and maintenance of food for the private sector that in turn will attract business. Some private investors have been running their own refrigerated rooms for more than ten years under international regulation certifications and have gained experience in the technology of food storage; they represent a potential sector that may be interested in participating in this particular project. There are other possible applications using the same absorption principle and geothermal energy, such as district cooling, hydroponic growing of plants and refrigeration or cooling for other products than food that will be left out of the present study. 2. AGRICULTURAL AND INDUSTRIAL SECTOR IN EL SALVADOR El Salvador, like any other tropical country in the Central American region, has a variety of food production, including meat, fruits and crops. Potential users for refrigeration facilities are therefore found in this sector. The need for refrigeration also includes the ability to preserve food when there are delays between delivery and consumption in the national and international markets. Figure 4 shows the level of production of different meat for the Central American region for 2005 (FAO, 2005a).

27; 1%

1694; 48%

972; 27%

870; 24%

Imported GWh

Thermal GWh

Hydro GWh

Geothermal GWh

FIGURE 2: Electricity generation by resource in GWh for El Salvador, Jan-Aug 2007

0

500

1,000

1,500

2,000

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

F

2008

F

2009

F

2010

F

2011

F

El Salvador

MW

COAL

COGEN

THERMAL

GEO

HYDRO

FIGURE 3: Forecast of installed electricity capacity by resource for El Salvador for 2007 to 2011

Report 4 27 Ábrego C.

FIGURE 4: Central America, various types of meat production in 2005

Regarding fish products, more than 40% of the national production is exported: alive, dry, chilled or frozen as shown in Figure 5 (FAO, 2005b). The national fish meat production comes from several sources, such as industrial fishing, local fishing and aquaculture, as shown in Table 1. The total import and export of fish meat for 2003 is shown in Table 2, in metric Tonnes. Imports cover 14% of the national total supply (FAO, 2005b).

TABLE 1: El Salvador fish production per source (metric Tonnes)

2000 2001 2002 2003 Industrial fishing 2,099 2,407 2,008 14,813 Marine handline fishing 4,566 5,044 12,007 11,038 Continental handline fishing 2,830 2,774 2,664 2,673 Aquaculture 260 395 782 1,130 Total production 9,755 10,620 17,461 29,654

-

20,000

40,000

60,000

80,000

100,000

120,000

Indigenous cattle meat Indigenous chicken meat Indigenous pigmeat

Prod

uctio

n [m

etri

c to

nnes

]

El Salvador

Honduras

Nicaragua

Costa Rica

Panama

$33,382,000 ; 57%

$25,239,426 ; 43%

Value for fish export USD

Gross value of fish production for national consumption USD

FIGURE 5: Gross value of fish production in El Salvador in 2003


TABLE 2: El Salvador fish supply in 2003 (metric Tonnes)

Production [Tonnes]

Export [Tonnes]

Import [Tonnes]

Total supply [Tonnes]

Supply/Capita[Tonnes]

Fish for alimentation 29,477 12,435 2,884 19,926 3.36 Fish for other purposes 118.7 - 4,697 Live fish 580.0 5 156

Along with fish, vegetables, aquatic invertebrates, bovine animals, that to some extent require refrigeration, are exported as shown in Figure 6 (ITC, 2005). Aquatic invertebrates are a main volume of product exportation; in 2004 exports exceeded 185% that of fish. Vegetables are also getting more important, increasing in both production and exportation. In 2004, exportation increased by 184% over that of 2003. This commodity may though need less refrigeration than the rest.

FIGURE 6: El Salvador export history for various frozen or preserved products (thousand USD)

Figure 7 shows some typical agricultural products for the tropical climate zone (FAO, 2005a). If cooling using an absorption system is found to be feasible as a part of a growing facility, the production could face changes by including untraditional products that until now are imported. But for this particular case further studies have to be carried out to estimate the necessary conditions for cooling a plant during growth. 3. COOLING AND REFRIGERATING FUNCTIONS As was mentioned previously, the need for cooling and refrigeration is connected to the ability for slowing down product decomposition in order to allow for production, transportation, and storage. Decomposition is, by definition, the result of bacterial and fungal action on a biological media, which in turn feeds on dead organisms. There are methods for preventing food decomposition. Some of them date from ancient times. These include drying food, and using specific chemicals added to water, such as potassium nitrate or sodium nitrates to lower the temperature. Drying food involves reducing the amount of water contained in the fibres of the food, needed by the decomposing organisms, by evaporation. Refrigeration freezes the water contained in the food fibres, thus slowing the growth of bacteria. Table 3 shows storage times for refrigerated foods (ASHRAE, 2002).

0 5,000 10,000 15,000 20,000 25,000

011 - Meat of bovine animals, fresh, chilled or frozen

012 - Other meat and edible meat offal, fresh, chilled or frozen (except meat and meat offal unfit or unsuitable for human consumption)

034 - Fish, fresh (live or dead), chilled or frozen

036 - Crustaceans, molluscs and aquatic invertebrates, whether in shell or not, fresh (live or dead), chilled, frozen, dried, salted or in brine;

crustaceans, in

054 - Vegetables, fresh, chilled, frozen or simply preserved (including dried leguminous vegetables); roots, tubers and other edible vegetable

products, n.e.s.,

Value 2004 Value 2003 Value 2002 Value 2001


TABLE 3: Sample of storage time for refrigeration of food

Product Storage

temperature [°C]

Relative humidity

[%]

Approx. storage lifea Product

Storage tempera-ture [°C]

Relative humidity

[%]

Approx. storage lifea

FISH MEAT (miscell.)Haddock, cod, perch -0.5 - 1 95 - 100 12 days Rabbits, fresh 0 - 1 90 - 95 1 - 5 days Hake, whiting 0 - 1 95 - 100 10 days DAIRY PROD.Halibut -0.5 - 1 95 - 100 18 days Butter 0 75 - 85 2 - 4 weeksHerring, kipper 0 - 2 80 - 90 10 days Butter, frozen –23 70 - 85 12 - 20 mo.Herring, smoked 0 - 2 80 - 90 10 days Cheese, cheddar Mackerel 0 - 1 95 - 100 6 - 8 days Long storage 0 - 1 65 12 months Menhaden 1 - 5 95 - 100 4 - 5 days Short torrage 4 65 6 months Salmon -0.5 - 1 95 - 100 18 days Processed 4 65 12 months Tuna 0 - 2 95 - 100 14 days Grated 4 65 12 months Frozen fish –30 - –20 90 - 95 6 - 12 months Ice cream, 10% fat –30 to –25 90 - 95 3 - 23 mon.SHELLFISHA Premium –35 to –40 90 - 95 3 - 23 mon.Scallop meat 0 - 1 95 - 100 12 days Milk Shrimp -0.5 - 1 95 - 100 12 - 14 days Fluid, pasteurized 4 - 6 7 days

Lobster American In seawater indefinitely Grade A (3.7%) 0 - 1 2 - 4 months

Oysters, clams (meat and liquid) 5 - 10 100 5 - 8 days Raw 0 - 4 2 days

Dried, whole 21 Low 6 - 9 monthsOyster in shell 0 - 2 95 - 100 5 days Dried, non fat 7 - 21 Low 16 months Frozen shellfish 5 - 10 90 - 95 3 - 8 mo. Evaporated 4 24 months BEEF Evapor. unsweet. 21 12 months Beef, fresh, average –2 - 1 88 - 95 1 week Condens., sweet. 4 15 months Beef carcass Whey, dried 21 Low 12 months Choice, 60% lean 0 - 4 85 - 90 1 - 3 weeks EGGS Prime, 54% lean 0 - 1 85 1 - 3 weeks Shell –1.5 - 0b 80 - 90 5 - 6 monthsSirloin cut (choice) 0 - 1 85 1 - 3 weeks Shell, farm cooler 10 - 13 70 - 75 2 - 3 weeksRound cut (choice) 0 - 1 85 1 - 3 weeks Frozen, Dried, chipped 10 - 15 15 6 - 8 weeks Whole –20 1 year plusLiver 0 90 5 days Yolk –20 1 year plusVeal, lean –2 - 1 85 - 95 3 weeks White –20 1 year plusBeef, frozen –20 90 - 95 6 - 12 months Whole egg solids 1.5 - 4 Low 6 - 12 mon.

-

10,000

20,000

30,000

40,000

50,000

60,000

70,000

Avocados Bananas Lemons and limes

Oranges Tomatoes Watermelons

Prod

uctio

n [m

etri

c ton

nes]

El Salvador

FIGURE 7: Production of various typical agricultural items for El Salvador in 2005 (metric Tonnes)


PORK Yolk solids 1.5 - 4 Low 6 - 12 months

Pork, fresh, average 0 - 1 85 - 90 3 - 7 days Flake album.solids Room Low 1 year plus

Pork,carcass,47% lean 0 - 1 85 - 90 3 - 5 days Dry spray albumen solids Room Low 1 year plus

Pork, bellies,35% lean 0 - 1 85 3 - 5 days CANDY Pork,fatback,100% fat 0 - 1 85 3 - 7 days Milk chocolate –20 - 1 40 6 - 12 mon.Pork, shoulder, 67% lean 0 - 1 85 3 - 5 days Peanut brittle –20 - 1 40 1.5 - 6 mon.

Pork, frozen –20 90 - 95 4 - 8 months Fudge –20 - 1 65 5 - 12 mon.Ham, 74% lean 0 - 1 80 - 85 3 - 5 days Marshmallows –20 - 1 65 3 - 9 monthsHam, light cure 3 - 5 80 - 85 1 - 2 weeks MISCELLANEOUSHam, country 10 - 15 65 - 70 3 - 5 months Alfalfa meal –20 70 - 75 1 year plusHam, frozen –20 90 - 95 6 - 8 months Beer, keg 1.5 - 4 3 - 8 weeksBacon, med. fat class 3 - 5 80 - 85 2 - 3 weeks Beer, bottles& cans 1.5 - 4 65 or less 3 - 6 monthsBacon,cured,farm sty. 16 - 18 85 4 - 6 months Bread –20 3 - 13weeksBacon, cured, packer style 1 - 4 85 2 - 6 weeks Canned goods 0 - 15 70 or

lower 1 year

Bacon, frozen –20 90 - 95 2 - 4 months Cocoa 0 - 4 50 - 70 1 year plusSausage, links or bulk 0 - 1 85 1 - 7 days Coffee, green 1.5 - 3 80 - 85 2 - 4 monthsSausage,coun.smoked 0 85 1 - 3 weeks Fur and fabrics 1 - 4 45 - 55 Sev. years Frankfurters,average 0 85 1 - 3 weeks Honey 10 1 year plus Polish style 0 85 1 - 3 weeks Hops –2 - 0 50 - 60 Sev. monthsLAMB Lard (without

antioxidant) 7 90 - 95 4 - 8 monthsFresh, average –2 - 1 85 - 90 3 - 4 weeks Choice, lean 0 85 5 - 12 days Maple syrup 0 90 - 95 12 - 14 mo.Leg, choice, 83% lean 0 85 5 - 12 days Nuts 0 - 10 65 - 75 8 - 12 mon.Frozen –20 90 - 95 8 - 12 months Oil, vegetable,salad 21 1 year plusPOULTRY Oleomargarine 1.5 60 - 70 1 year plusPoultry, fresh, average –2 - 0 95 - 100 1 - 3 weeks Orange juice –1 - 1.5 3 - 6 weeksChicken, all classes –2 - 0 95 - 100 1 - 4 weeks Popcorn, unpopped 0 - 4 85 4 - 6 weeks

Turkey, all classes –2 - 0 95 - 100 1 - 4 weeks Yeast, baker’s compressed -0.5 - 0

Turkey breast roll –4 - –1 6 - 12 months Tobacco, hogshead 10 - 18 50 - 65 1 year Turkey frankfurters –20 - –10 6 - 16 months Bales 2 - 4 70 - 85 1 - 2 years Duck –2 - 0 95 - 100 1 - 4 weeks Cigarettes 2 - 8 50 - 55 6 months Poultry, frozen –20 90 - 95 12 months Cigars 2 - 10 60 - 65 2 months 4. DESCRIPTION OF BASIC CONCEPTS OF ABSORPTION SYSTEMS 4.1 Ammonia and water absorption system The whole thermodynamic process for an ammonia refrigeration absorption cycle is shown in Figure 8, based on Ólafsson (2007). In Figure 8, the x value refers to the ammonia content mixed with water, while T is the temperature of the mixture. For a given pressure, the bubble and dew curves are given; the bubble curve refers to the temperature at which the first drop of liquid will start to boil, and the dew curve shows the temperature at which the first condensate drop will start to be formed in the vapour phase. FIGURE 8: Ammonia refrigeration

absorption cycle diagram

0 0.2 0.4 0.6 0.8 1-50

-5

40

85

130

175

x

T [°

C]

High P dew High P bubble Low P dew Low P bubble

Geothermal water

Cooling water Cooling water


FIGURE 9: Typical absorption cycle components

Figure 9 shows a typical absorption cycle component diagram where the interconnections between the pieces of equipment can be observed. The principles of the various components are described below: Heat exchangers: To calculate the heat exchanger area, Equation 1 is used:

∆ln

1

where Q = Heat transfer in heat exchanger [W];

U = Overall heat transfer coefficient [W/(m2 ºC)]; A = Area [m2]; Th1 = Input temperature of hot fluid [ºC]; Th2 = Output temperature of hot fluid [ºC]; Tc2 = Input temperature of cold fluid [ºC]; Tc1 = Output temperature of cold fluid [ºC].

The estimated U values for the heat exchanger are shown in Table 4:

TABLE 4: Assumed U values for heat exchangers [kW/m2K]

Heat exchangers U - value [kW/(m2K)] Type

Condenser 1.1 Shell & tube Solution hx. Thermal compr.) 1.8 Plate hx Solution hx. (NH3 superheat) 0.2 Plate hx Absorber 0.785 Shell & tube Desorber 1.2 boiling / 0.8 heating Shell & tube Rectifier 0.2 Shell & plate Evaporator 1.2 Shell & tube

Desorber

RectifierCondenser

Evaporator

Absorber

Solution heat exchanger(thermal compressor)

Solution heat exchanger (superheat)

Pump

Pump

Throttling valve Throttling

valve

Hot brine


Desorber generator: In the desorber or generator, the heat source exchanges heat with the working fluid: ammonia - water or Lithium Bromide – water mixture, as shown in Figure 10. The heat gained by the working fluid is used to evaporate the refrigerant substance while most of the carrying fluid remains liquid due to the evaporation tempera-ture difference.

Once the refrigerant changes in phase, the vapour and liquid are separated into two different pipes with the vapour connected to the rectifier and the liquid connected to the pre-heater and the absorber. When the mixture is heated, the vapour reaches an intermediate refrigerant concentration, whereas the liquid reduces its refrigerant concentration. The general equations are the following (Björnsdóttir, 2004): The energy balance:

(2) the ammonia balance:

(3) and the mass balance:

(4) where m = Mass [kg]; h = Enthalpy [kJ/kg]; Q = Energy [kJ]); x = Vapour fraction. Rectifier: The vapour coming from the absorber does not work well as a refrigerant because it still has some water content. Therefore, the fluid is passed into the rectifier, to cool and condense the remaining water, becoming a richer, almost pure, solution in the process. The condensed water is collected and sent back to the desorber. Equations 5 and 6 show the energy and mass balance, respectively, for this process:

(5)

(6)

FIGURE 10: Generator or desorber, schematic diagram for the ammonia reference cycle


Condenser: The conden-ser is a heat exchanger in which the refrigerant coming from the Desorber is cooled down to liquid form by rejecting heat to the cooling media (cooling water that may come from a river or a cooling tower) as shown in Figure 11. The vapour refrigerant is transformed into liquid in order to extract more heat from the environment to be cooled or refrigerated, and to achieve a lower ambient temperature. The condenser is then connected to a heat exchanger and a throttle valve. The pinch point in the condenser heat exchanger can be located either on the cold end where refrigerant vapour enters the equipment or at the point where the refrigerant starts to condense. The energy balance equation for the condenser is given according to Equation 7:

(7)

Figure 12 shows the temperature profile of the refrigerant in the condenser. The throttling valve is located between the condenser and the evaporator. Its main function is to drop the pressure in the liquid refrigerant and, thus, reduce its boiling temperature which allows for the heat exchange between fluid and refrigeration space. The evaporator is a unit where the refrigerant takes heat from a secondary fluid which is in direct contact with equipment in the room being refrigerated or cooled. At one point, it receives low-temperature and low-pressure liquid refrigerant which, when heated, changes into vapour and leaves the equipment. At another point, it receives the secondary fluid leaving the facility; when it passes the evaporator, it is cooled and returns to the same facility to maintain certain ambient conditions. The evaporation process could be at constant temperature and pressure, or present a slight temperature difference when there is a presence of another substance in the refrigerant.

0

5

10

15

20

25

30

35

0 20 40 60 80

Tem

pera

ture

[°C

]

Area [m2]

Refrigerant temperature [°C] Cooling temperature [°C]

FIGURE 12: Condenser temperature profile

0 0.2 0.4 0.6 0.8 1-50

-5

40

85

130

175

x

T [°

C]

High P dew High P bubble Low P dew Low P bubble

Geothermal water

Cooling water Cooling water

Cooling water

FIGURE 11: The thermodynamic cycle for ammonia (condenser)


There are two types of evaporators, the flooded and the direct type. Special care must be taken in the selection of evaporators due to problems related to leakages of refrigeration liquid into the compressor or leakages from the oil compressor into the refrigerant. The energy balance equation in the evaporator is according to Equation 8:

(8) In the absorber the refrigerant is absorbed back into the carrying fluid, in order to carry the refrigerant in a liquid form from a low-pressure state to a high-pressure state. In the heat exchanger, two different fluid flows take place: vapour refrigerant coming from the evaporator, and weak liquid carrying the solution coming from the generator. Cooling fluid is also passed through the exchanger. In this case, the outlet cooling water coming from the condenser, being some degrees hotter, will then be used to cool the fluid in the absorber. The cooling part of the absorber is needed in order to eliminate heat arising while condensing the vapour refrigerant, and to achieve a higher concentration of ammonia in the rich solution. If the condensing sink temperature supplied is not low enough, it will result in a lower concentration of the rich solution and the efficiency of the whole cycle will be reduced. The energy balance in the absorber is:

(9) the ammonia balance is:

(10) and the mass balance is:

(11) The pump and the throttling valve: The pump increases the pressure of the rich solution coming from the absorber at low temperature, and sends it to the rectifier, solution heat exchanger and desorber. The throttling valve reduces the pressure of the weak solution from the high-pressure equipment to the pressure of the absorber. The solution heat exchanger receives the weak solution coming from the desorber and going to the throttling valve on one side, and on the other the rich solution coming from the rectifier and going to the desorber. Its function is to increase the coefficient of performance (COP). 4.2 Lithium-Bromide water cooling absorption cycle The cooling absorption cycle works using the same principle as the absorption refrigeration cycle. The main difference is that in this cycle, water is used as the refrigerant, not as the carrying fluid. Because of this, if no antifreeze is added to the water, the minimum cooling temperature that the cycle can achieve is the freezing temperature of water. Lithium Bromide solution works as the carrying fluid and has a higher boiling temperature than water. Neither the rectifier nor the solution heat exchanger between the condenser and evaporator are needed in the system as shown in Figure 13.

Similar equations apply for the components. The desorber or generator balances are shown in Equations 12, 13 and 14:


(12)

(13)

(14) Figure 14 shows the temperature profile for the generator in the cooling absorption cycle.

FIGURE 14: Desorber temperature profile for the Lithium Bromide cooling cycle

100 150 200 250 300 350 40060

82

104

126

148

170

2625

2665

2705

2745

2785

2825

hlb[kJ/kg]

T lb

[°C

]

Tlb [°C]

Tbrine [°C]

hVlb [°C]hVlb [°C]

hVlb

[kJ/

kg]

Absorber

|

Generator

Condenser

Evaporator

Pump


valve

Solution heat exchanger

FIGURE 13: Absorption cooling cycle with Lithium Bromide system components


Condenser balance equations are given in Equations 15, 16 and 17:

(15)

(16)

(17)

Figure 15 shows the temperature profile for the condenser in the absorption cooling cycle.

FIGURE 15: Condenser temperature profile

The evaporator balance equations are given in Equations 18 and 19:

(18)

(19)

And the absorber balance equations are given in Equations 20, 21 and 22:

(20)

(21)

(22)

-2.5 -0.0 2.5 5.0 7.5 10.0 12.50

100

200

300

400

500

600

700

s [kJ/kg-K]

T [°

C]

110 bar 55 bar 25 bar

6.22 bar

1 bar

0.2 0.4 0.6 0.8

0.0

076

0.2

3 1

.25

6.8

8 4

0 2

05 m

3/kg

T[i]

Cooling water

Refrigerant water


5. RESULTS

5.1 Project objective The purpose of the project is to estimate the cost of installing and operating an absorption refrigeration system for the Ahuachapán and Berlín geothermal fields in El Salvador using geothermal brine as the heat source with ammonia and water as the carrying fluid, and using it to refrigerate beef (sample product) on the site of the power plant. A technical evaluation of the cooling cycle using geothermal brine as a heat source with a Lithium Bromide water mixture as the carrying fluid with no specific product will be included for both geothermal fields. The cooling load will just be assumed. 5.2 Berlín and Ahuachapán field fluid characteristics Berlín geothermal field generates 94 MWe gross, but a new binary power station is under construction which will increase the gross generation by 9 MW. The field is producing with 13 wells and reinjection into 16 wells. The mass flow of the field is shown in Figure 16 (LaGeo S.A. de C.V., 2007). The production drop in Figure 16 is due to the fact that unit 3 was under maintenance, and the wells that feed this unit were closed.

FIGURE 16: Berlín geothermal field mass flow production [kg/s]

So far all the water is being reinjected into the field at separation temperature, but when the new binary plant goes into operation, a brine mass flow of about 382 kg/s will be directed to the heat exchangers of the binary plant and its temperature will drop from 180 to 140°C which, due to silica deposition, is the minimum working temperature for the reinjection system. Hence, the available geothermal water that can be taken into account for the absorption refrigeration system is around 187 kg/s at a separation temperature of 180°C, shown in Figure 17. The experiments carried out on the Berlín geothermal brine for silica deposition are summarized in Table 5 (Molina et al., 2005). They

0

100

200

300

400

500

600

700

Mas

s flo

w [k

g/s]

Total reinjected flow [kg/s] Total steam flow [kg/s]


show that dosing with HCl to maintain a pH between 5.5 and 6, the temperature of 140°C gives a safe lower value for reinjection.

TABLE 5: Berlín experimental tests for silica deposition

Rates for

tubing size [“)

Brine treatment option

Silica deposition range [g/Tonne]

From To

1/4

HCl, pH 5.0 1 at 150ºC 1 at 90ºC HCl, pH 5.5 1 at 150°C 3 at 90ºC Millisperse 5 ppm 3 at 120ºC 3 at 65ºC Drew 20 ppm 1 at 140ºC 5 at 100ºC Millisperse 10 ppm 1 at 140ºC 12 at 90ºC Drew 10 ppm Insufficient data to assess

3/4

HCl, pH 5.0 1 at 150ºC 1 at 90ºC HCl, pH 5.5 1.5 at 150ºC 1.5 at 65ºC Millisperse 5 ppm 1 at 120ºC 2 at 65ºC Drew 20 ppm 1 at 140ºC 3 at 100ºC Millisperse 10 ppm 1 at 140ºC 4 at 100ºC Drew 10 ppm Insufficient data to assess

The Ahuachapán geothermal field is located in the western part of El Salvador. There are 16 production wells and 4 reinjection wells for steam and water production, which is shown in Figure 18 (LaGeo S.A. de C.V., 2007). Ahuachapán geothermal field is experiencing an increase in its mass flow production due to project “Ahuachapán Optimization”. This has meant drilling of more production wells in order to replace the present production wells with new ones, and shift the reservoir production zone. Ultimately, it will also result in some increase in the total mass flowrate due to larger diameter production casings in the new wells. Ahuachapán geothermal power station is also expected to have its own binary cycle. Some part of the separated geothermal water need to go directly to the binary plant heat exchangers without going to the second flash process, as shown in Figure 19. Subsequently, all the water in Ahuachapán geothermal field will be used for electricity production. The separation temperature of the water will then be reduced from about 165°C to about 95°C. If a cooling or refrigeration cycle were to be used in Ahuachapán field, it would have to be supplied from wells currently producing electricity, and care must then be taken to evaluate the reservoir response to extra mass extraction.

382; 67%

187; 33%Reinjection UI, U2 [kg/s]

Reinjection U3 [kg/s]

FIGURE 17: Berlín reinjection distribution [kg/s]


FIGURE 18: Ahuachapán mass flow production [kg/s]

Silica deposition tests shown in Table 6 (Guerra et al., 2005), revealed that for the Ahuachapán geothermal brine being cooled down to 90°C, it would require acid dosing. This temperature will be considered a limit for the use of any cooling or refrigerating cycle.

5.3 Cooling load example for refrigeration application For the case of a refrigerating load for a typical chilling process above freezing, calculations were taken from the ASHRAE refrigeration handbook (ASHRAE, 2002). It included the inputs and assumptions for the product to be chilled, trucks with offal, chilled from 38 down to 1°C within 2 hours, as shown in Table 7. The heat gain from the room surfaces is calculated with Equation 23.

∆ (23) where q = Heat gain through walls, floors and ceiling [kW];

A = Outside area of section [m2]; ΔT = Temperature difference between outside air and air of the refrigerated space [°C]; U = Overall heat transfer coefficient [W/m2 K].

0

100

200

300

400

500

600

700

800

Mas

s flo

w [k

g/s]

Steam flow [kg/s] Water flow [kg/s]

Unit 2 end maintenance8/4/07-5/5/07

Unit 3 start maintenance

1/7/07

Unit 3 end maintenance

23/8/07

Well AH-35C in production

25/7/07

Well AH-35C out of production 9/8/07

520; 78%

151; 22%Flasher flow [kg/s]Binary flow [kg/s]

FIGURE 19: Ahuachapán brine use distribution [kg/s]


TABLE 6: Ahuachapán silica deposition trials

Tubing size [“]

Brine treatment option

Silica deposition rates range [g/Tonne]

From To

1/4

Untreated brine 0.21 to 105°C 1.34 to 60°C HCL to pH = 6.0 1.01 to 90°C 0.16 to 80°C HCL to pH = 5.5 0.55 to 90°C 0.2 to 80°C Millisperse 5 ppm 0.24 to 90°C 0.22 to 80°C Millisperse 10 ppm 0.26 to 90°C 1.09 to 80°C Drew 10 ppm 0.25 to 90°C 0.07 to 80°C Drew 20 ppm 0.23 to 90°C 0.15 to 80°C

3/8

Untreated brine 0.23 to 105°C 0.15 to 60°C HCL to pH = 6.0 1.11 to 90°C 0.21 to 80°C HCL to pH = 5.5 0.35 to 90°C 0.21 to 80°C Millisperse 5 ppm 0.02 to 90°C 0.32 to 80°C Millisperse 10 ppm 0.54 to 90°C 0.04 to 80°C Drew 10 ppm 0.15 to 90°C 0.17 to 80°C Drew 20 ppm 0.29 to 90°C 0.66 to 80°C

TABLE 7: Refrigeration load calculation assumptions

Refrigeration assumptions

Product average initial temperature [ºC] 38 Product average final temperature [°C] 1 Room temperature [°C] -18 Outdoor temperature [ºC] 26 Outdoor RH [%] 70 Room area [m2] 2500 Volume of the room [m3] 10000 Room air changes per 24 hours 12 Room gain [W/m2K] 0.426 Time to refrigerate [hours] 2

The overall heat transfer coefficient U is calculated with Equation 24:

(24)

where x = Wall thickness [m]; k = Thermal conductivity of wall material [W/m K]; h = Inside surface conductance [W/m2 K]; ho = Outside surface conductance [W/m2 K].

The surface conductance is assumed the same as for still air. The room gain the U value in W/m2K is given, not calculated. For the amount of mass for the offal and truck, the results are shown in Table 8. The heat gain from air infiltration is calculated with Equation 25:

(25) where hi = Enthalpy of infiltration air [kJ/kg];

hr = Enthalpy of refrigerated air [kJ/kg]; V = Volume of the refrigerated space [m3]; RAC = Room air changes in 24 hours.


TABLE 8: Mass conversions for the offal and truck sample

Mass conversions kg of meat/truck 330 kg of truck 180 Number of trucks 34 Time to refrigerate [hours] 2 kg/s of meat 1.558kg/s of truck 0.85

The change in enthalpy can be calculated from the psychrometric chart, shown in Figure 20, with the temperatures of the refrigerated air and the outdoor air, considering that the relative humidity of the air inside the room is 100%. The results of the calculation are shown in Table 9.

TABLE 9: Result of volumetric energy

calculation

From psychrometric chart Heat from infiltration air [kJ/m3] 95.3

Other assumptions are related to the heat gained for the electrical motors and lighting system. The values are shown in Table 10.

TABLE 10: Assumption for the electrical motors and lighting system

Assumptions Fan and motor [kW] 7.5 Light [kW] 0.2

The general formula for the refrigeration load is shown in Equation 26:

(26) The definitions are given in Table 11, which shows the results of the calculation.

TABLE 11: Heat load calculation results

qr Refrigeration load [kW] 414.5 mm Mass flow of meat [kg/s] 1.6 mt Mass flow of trucks [kg/s] 0.9 cm Specific heat of meat [kJ/kg K] 3.14 ct Specific heat of truck, containers or platforms (0.5 for steel) [kJ/kg K] 0.5 t1 Average initial temperature [ºC] 38 t2 Average final temperature [ºC] 1 qm Heat gain from the meat [kW] 181.0 qt Heat gain from the trucks [kW] 15.725 qw Heat gain through room surfaces [kW] 46.86 qi Heat gain through infiltration [kW] 132.36 qm Heat gain from equipment and lighting [kW] 38.5 tm Time to refrigerate [hours] 2

-20 -10 0 10 20 300.000

0.005

0.010

0.015

0.020

0.025

0.030

T [°C]

Hum

idity

ratio

Pressure = 1.0 [bar]

10°C

15°C

20°C

25°C

30°C

0.2

0.4

0.6

0.8

0.8

0.825

0.85 m3/kg

0.875 0.9

FIGURE 20: Psychrometric chart


It is always a recommended to have a safety factor of 20% over the estimated value, to allow for the possible discrepancies between the actual and design situations; the final results are shown in Table 12.

TABLE 12: Cooling calculation with safety factor

Factor used for computing refrigeration load [%] 20

Refrigeration load, qr [kW] 497.4 5.4 Thermodynamic absorption cycles for the Berlín and Ahuachapán geothermal fields Figure 21 shows the thermodynamic processes for the Berlín geothermal field for the ammonia-water case, while Figure 22 shows the cooling and refrigeration process for the Ahuachapán geothermal field.

FIGURE 21: Berlín ammonia water absorption refrigeration process

Figure 23 shows that due to the tropical environmental conditions, the COP was reduced to very low values when trying to increase the cooling temperature for the condenser and absorber for either Berlín or Ahuachapán. For conditions in El Salvador, a cooling water temperature higher than 28-29°C will be required, assuming that a cooling tower is used. Therefore, a cooling absorption refrigeration cycle in combination with an ammonia refrigeration cycle is proposed in order to provide the low cooling water temperature required. A schematic configuration is shown in Figure 24. It is important to take into consideration the following assumptions:

• The cost of the cooling absorption refrigeration cycle is not included in the cost of operation and installation; it will have to be added on in further calculations.

m23 = 60 [kg/s]

t23 = 16 [C]

econshx = 0.2642

p12 = 105 [kPa]

m27 = 512 [kg/s]T16 = -25 [C]

Qe = 500 [kW]

x9 = 0.999

T19 = 180 [C]

m19 = 150 [kg/s]

eshx = 0.8

dx = 0.05

Maximumutil ization,hours = 2500 [hr]

Pricehot,water = 0.01001 [US$/(K*m3 )]

Priceelectricity = 0.09719 [US$/kWh]

Pricecold,water = 0.07363 [US$/m3]

pincha = 5 [C]

Qd = 1926 [kW]

Qc = 510.6 [kW]

Qa = 1347 [kW]

Acondenser = 80.12 [m2]

T24 = 18.04 [C]

t10 = 22 [C]m10 = 0.4299 [kg/s]

p10 = 927.3 [kPa]

T18 = -13.3 [C]

Qconshx = 13.55 [kW]

T12 = -32.6 [C]x12 = 0.999 m14 = 0.06856 [kg/s]

m16 = 0.3598 [kg/s]

m15 = 0.001538 [kg/s]

Aevap = 94.46 [m2]

T27 = -20 [C]

T28 = -19.03 [C]

T17 = -25.97 [C]

T15 = -25 [C]

T14 = -32.6 [C]

T11 = 15.35 [C]

T1 = 23.04 [C]X2 = 0.3398T2 = 23.21 [C]

Qrect = 327.4 [kW]

Qhx = 1333 [kW]

t8 = 35.27 [C]T7 = 97.38 [C]

T3 = 64 [C]T4 = 102.6 [C]

Ashx = 49.15 [m2]

Atot,desorber = 22.42 [m2]

Atot,rect = 54.04 [m2]

T5 = 48.99 [C]

T22 = 35.57 [C]

T9 = 35.27 [C]

T6 = 36.3 [C]

T20 = 177.1 [C]

T24 = 18.04 [C] T25 = 23.41 [C]

m1 = 6.097 [kg/s]

ieff = 0.08

COP = 0.2582

Iannual,cost = 178924

Pricecold,water2 = 1

Iannual,average = 429044

Iannual,cost2 = 679164

δT = 5

m13 = 0.3613 [kg/s]

Aconshx = 1.773

anUS = 82719

an = 5.617E+06


FIGURE 22: Ahuachapán ammonia refrigeration process

• The cost of installing and operating the absorption refrigeration cycle is estimated for temperatures of T = 8 and 16°C.

• It is important to note that the evaporator heat load and evaporator temperatures for the cooling

cycle will not be in correspondence with those of the refrigeration cycle for the condenser and absorber. Further calculations will be needed to continue with this option. The Lithium Bromide cooling cycle will only address the basic concepts of the system for an assumed case.

m23 = 60 [kg/s]

t23 = 16 [C]

econshx = 0.7

p12 = 65 [kPa]

m27 = 512 [kg/s]T16 = -25 [C]

Qe = 750 [kW]

x9 = 0.999

T19 = 160 [C]

m19 = 50 [kg/s]

eshx = 0.8

dx = 0.05

Maximumuti lization,hours = 2500 [hr]

Pricehot,water = 0.01001 [US$/(K*m3 )]

Priceelectricity = 0.09719 [US$/kWh]

Pricecold,water = 0.07363 [US$/m3]

pincha = 5 [C]

Qd = 4481 [kW]

Qc = 733.3 [kW]

Qa = 2735 [kW]

Acondenser = 125.6 [m2]

T24 = 18.92 [C]

t10 = 22 [C]m10 = 0.6174 [kg/s]

p10 = 927.3 [kPa]

T18 = 7.181 [C]

Qconshx = 48.87 [kW]

T12 = -41.8 [C]x12 = 0.999 m14 = 0.09366 [kg/s]

m16 = 0.5223 [kg/s]

m15 = 0.001431 [kg/s]

Aevap = 62.65 [m2]

T27 = -20 [C]

T28 = -18.54 [C]

T17 = -27.4 [C]

T15 = -25 [C]

T14 = -41.8 [C]

T11 = 5.152 [C]

T1 = 23.92 [C]

X2 = 0.2714

T2 = 24.1 [C]

Qrect = 1203.9 [kW]

Qhx = 2052 [kW]

t8 = 35.27 [C]T7 = 113.3 [C]

T3 = 60 [C]T4 = 118.7 [C]

Ashx = 79.73 [m2]

Atot,desorber = 92.80 [m2]Atot,rect = 203.5 [m2]

T5 = 66.45 [C]

T22 = 53.38 [C]

T9 = 35.27 [C]

T6 = 41.48 [C]

T20 = 139.2 [C]

T24 = 18.92 [C] T25 = 29.83 [C]

m1 = 9.602 [kg/s]

ieff = 0.08

COP = 0.1668

Iannual,cost = 296417

Pricecold,water2 = 1

Iannual,average = 546537

Iannual,cost2 = 796657

δT = 5

m13 = 0.5237 [kg/s]

Aconshx = 10.84

anUS = 140512

an = 9.542E+06

FIGURE 23: Refrigeration COP sensibility with respect to the cooling temperature at condenser and absorber;

econshx = Effectiveness of the condenser solution for the heat exchanger

12.6 12.7 12.8 12.9 13 13.1 13.2 13.3 13.4 13.50.295

0.3

0.305

0.31

0.315

0.32

0.325

0.33

Temperature cooling water [°C]

CO

P

COP econshx = 0.3

econshx = 0.56


FIGURE 24: Proposed configuration for providing low water temperature in tropical climate 5.5 Alternatives for location of the cooling facility The absorption cooling system should be located in the vicinity of the power station or some distance away from it in order to provide better access to mains roads for easier transportation of products. The case is only analyzed for the Lithium Bromide absorption system where the refrigerant substance is water instead of ammonia for the refrigeration cycle. The first option is to have the condenser and the evaporator away from the power station, as shown in Figure 25, at a site close to main access roads. This location has the following implications:

• A second cooling tower must be installed in the cooling facility; the cooled water will be used for the condenser;

• A network of pipelines carrying steam at very low pressure will be part of the facility installation and operation;

• A pump could be used in the system to overcome pressure drop in the steam pipeline from the power station to the cooling storage facility.

The second option is to have all the equipment at the power station site, and a heat exchanger in the cooling storage site facility for cooled water, as shown in Figure 26. This option has the following implications:

• The same cooling tower for the condenser and the absorber; • A pump might be required to overcome the pressure drop in the cooled water pipeline from

the power station to the storage cooling facility some kilometres away; • A well insulated network of pipelines for carrying cooled water must be part of the installation

and operation.

Propose of a combined solution with Li-Brand NH3 systems.Continue with refrigeration partial costestimation plus a cost sensibility analysisrespect to cooling temperature.Cooling cost to be out of the estimation.

COP

Li‐Br.NH3

Combined cooling and refrigeration cycle


FIGURE 25: First case with condenser and evaporator located away from the power station

FIGURE 26: Second case with all the main cooling equipment at the power station site To evaluate the conditions for both scenarios, the same program in EES - Engineering Equation Solver (F-Chart Software, 2004) was used for the evaluation of the technical aspects for the Lithium Bromide cooling cycle, and a pressure drop in the pipelines added to the calculation. Assumptions considered in the pressure drop calculation include:

• There is no change in the level of the terrain; i.e. all is flat to avoid including the piezometric pressure part of the Bernoulli equation;

• The pipes are well insulated and no temperature drop to the environment is taken into account; • The Colebrook-White equation was used for the calculation of the friction factor of the pipes

(F-Chart, Software, 2004). Results from the EES calculations for the Berlín geothermal field program are shown in Figure 27 (Rogowska and Szaflik, 2005).


FIGURE 27: The Lithium Bromide cooling process for the Berlín field

Similarly, the results from the EES calculations for the Ahuachapán field are shown in Figure 28.

FIGURE 28: The Lithium Bromide cooling process for the Ahuachapán field

Tpinch,e = 6

Qe = 1873 [kW]

T17 = 30.39 [C]P17 = 3 [bar]

T21 = 29 [C]

T22 = 40.63 [C]P21 = 2 [bar]

tpinch,c = 10 [K]

P19 = 2 [bar]

T19 = 40.63 [C]T20 = 56 [C]

ηp = 0.8

P12 = 1 [bar]

m24 = 29 [kg/s]

XLB,14 = 45T13 = 90 [C]

ηp,net = 0.8

Tpinch,ghx = 2 [K]

T24 = 180 [C]

T25 = 150 [C]

QDG = 1804

P24 = 3 [bar]

m7 = 0.8 [kg/s]

m8 = 0.8 [kg/s]

m9 = 0.8 [kg/s]

m10 = 0.8 [kg/s]

m13 = 6 [kg/s]

m16 = 5.2 [kg/s]

m17 = 20 [kg/s]

m18 = 20 [kg/s]

m20 = 42 [kg/s]m19 = 42 [kg/s]

m21 = 42 [kg/s]m22 = 42 [kg/s]

T7 = 120.7 [C]

XLB,13 = 42

T9 = 2 [C]

H8 = 163.4 [kJ/kg]

H9 = 163.4 [kJ/kg]

T8 = 39 [C]

P7 = 1 [bar]

T18 = 8 [C]

COP = 0.4933

T10 = 2 [C]

T14 = 120.7 [C]

m14 = 5.2 [kg/s]h14 = 161.2 [kJ/kg]

H7 = 2718 [kJ/kg]

h13 = 201.5 [kJ/kg]

P14 = 1 [bar]

m11 = 6 [kg/s]

T11 = 11.88 [C]

Qshx = 55.25

h12 = 192.2 [kJ/kg]T12 = 11.88 [C]

P15 = 1 [bar]

T15 = 70.56 [C]h15 = 150.6 [kJ/kg]

Qabs = 2699

h16 = 150.6 [kJ/kg]

H10 = 2504 [kJ/kg]

h11 = 14.5 [kJ/kg]

P10 = 0.00706 [bar]

P9 = 0.00706 [bar]

P8 = 1 [bar]

H21 = 121.7 [kJ/kg]H22 = 170.3 [kJ/kg]

Qcon = 2044

P16 = 0.00706 [bar]T16 = 14.96 [C]

UDG = 1.2

X21 = -100

X17 = 0.0001108

X19 = -100

xlb,7 = 22.5

H[14]>H[15]T[14]>T[15]

X24 = 0

P31 = 3 [bar]

X7 = 100

X8 = 0.00003431

Pressure Drop

Absorber

|

Generator

Condenser

Evaporator

Pump


valve


Tpinch,e = 6

Qe = 1873 [kW]

T17 = 30.39 [C]P17 = 3 [bar]

T21 = 29 [C]

T22 = 40.63 [C]P21 = 2 [bar]

tpinch,c = 10 [K]

P19 = 2 [bar]

T19 = 40.63 [C]T20 = 56 [C]

ηp = 0.8

P12 = 1 [bar]

m24 = 30 [kg/s]

XLB,14 = 45T13 = 80 [C]

ηp,net = 0.8

Tpinch,ghx = 2 [K]

T24 = 160 [C]

T25 = 120 [C]

QDG = 2531

P24 = 3 [bar]

m7 = 0.8 [kg/s]

m8 = 0.8 [kg/s]

m9 = 0.8 [kg/s]

m10 = 0.8 [kg/s]

m13 = 6 [kg/s]

m16 = 5.2 [kg/s]

m17 = 20 [kg/s]

m18 = 20 [kg/s]

m20 = 42 [kg/s]m19 = 42 [kg/s]

m21 = 42 [kg/s]m22 = 42 [kg/s]

T7 = 120.7 [C]

XLB,13 = 42

T9 = 2 [C]

H8 = 163.4 [kJ/kg]

H9 = 163.4 [kJ/kg]

T8 = 39 [C]

P7 = 1 [bar]

T18 = 8 [C]

COP = 0.3633

T10 = 2 [C]

T14 = 120.7 [C]

m14 = 5.2 [kg/s]h14 = 273.1 [kJ/kg]

H7 = 2718 [kJ/kg]

h13 = 177.2 [kJ/kg]

P14 = 1 [bar]

m11 = 6 [kg/s]

T11 = 11.88 [C]

Qshx = 637.2

h12 = 71.03 [kJ/kg]T12 = 11.88 [C]

P15 = 1 [bar]

T15 = 70.56 [C]h15 = 150.6 [kJ/kg]

Qabs = 2699

h16 = 150.6 [kJ/kg]

H10 = 2504 [kJ/kg]

h11 = 14.5 [kJ/kg]

P10 = 0.00706 [bar]

P9 = 0.00706 [bar]

P8 = 1 [bar]

H21 = 121.7 [kJ/kg]H22 = 170.3 [kJ/kg]

Qcon = 2044

P16 = 0.00706 [bar]T16 = 14.96 [C]

UDG = 1.2

X21 = -100

X17 = 0.0001108

X19 = -100

xlb,7 = 22.5

H[14]>H[15]T[14]>T[15]

X24 = 0

P31 = 3 [bar]

X7 = 100

X8 = 0.00003431

Absorber

|

Generator

Condenser

Evaporator

Pump


valve



The results are addressed at the end of this main section. It can though be stated that the Lithium Bromide cycle for cooling uses low refrigerant pressures. Because of this, the specific fluid volume is large, resulting in large diameter pipelines which are more expensive. A cooled water gathering system results in smaller pipeline diameters and is therefore the better option. 5.6 Capital and operating cost estimations 5.6.1 The Berlín geothermal field The capital cost for equipment was calculated according to Equations 27, 28 and 29 (Ólafsson, 2007):

. (27)

0.01 . (28)

, ,α (29)

where I = Investment cost of equipment;

Co = Constant, indicating cost for each m2 of heat exchanging area; C1 = Constant, indicating cost for each 100 kW of pumping power; A = Heat exchanging area [m2]; P = Pumping power [kW]; CPE,Y = Cost for purchased equipment Y (as W); XY = For component involved, power in boilers, compressors, etc. α = For each piece of equipment, αevaporator = 0.54; αcompressor = 0.95; etc.

The CPE,Y formula is valid for the following ranges:

Evaporators: 10 - 1000 m2; Compressors: 0.05 - 8 MW.

The estimated values are:

Co = USD 736.2; for plate heat exchangers; Co = USD 1,104 .4; for shell and tube heat exchangers; C1 = USD 11,044.4; per 100 kW, not linear.

The capital costs were then estimated and are shown in Table 13. It is important to note that there are different scenarios evaluated and calculated due to the sensibility of the temperature at the condenser and absorber inlets on the total cost. El Salvador has warm climate conditions and this will limit the coefficient of performance operation of the refrigeration cycle.

The annual cost from investment is calculated with Equation 30:

, (30) where Anj = Annuity (capital cost) for the project;

CFC,j = Fixed cost for project; ieff = Annual effective rate of return; n = Number of years where the project is operated.


TABLE 13: Berlín field capital costs for the refrigeration cycle

Investment cost (8°C) 0.5 MW

[USD]


[USD] Condenser 37,459 37,312 Solution hx. (thermal compressor) 14,564 16,536 Solution hx. (NH3 superheat) 5,121 1,783 Absorber 74,894 83,927 Desorber 7,895 12,028 Rectifier 14,345 27,160 Evaporator 47,053 42,009 Pumps - - Thermal compressor pump 1,473 1,473 Evaporator bleed pump 1,473 1,473 Annual cost from investment – annual capital cost 204,277 223,701

The cost breakdown structure for the Berlín field is shown in Table 14.

TABLE 14: Berlín field, cost breakdown for the refrigeration cycle

I. Fixed Capital Investment (FCI) 0.5 MW power

T=8°C [USD]

0.5 MW power T=16°C [USD]

A. Direct cost (DC) 1. Onsite cost (ONSC) – Purchased equipment cost (PEC):

1. Heat Exchangers 201,329 220,755 2. Pumps 2,403 2,992 3. Pipes in system (5% of 1 +2) 10,187 11,187 4. Electrical control & monitoring system (30% of 1+2+3) 64,176 70,480

Total onsite cost: 278,095 305,414 2. Offsite cost (OFSC) 1. Civil structural and architectural work 20% of ONSC 55,619 61,083

2. Service facilities (hot source &codl. sink connection) (25% ONSC) 69,524 76,353

3. Contingencies (15% of ONSC) 41,714 45,812 Total direct cost (DC) 444,952 488,662 B. Indirect cost (IDC) 1. Engineering and supervision (15% of DC) 66,743 73,299

2. Construction cost including contractor´s profit (15% of DC) 66,743 73,299

3. Contingencies (20% of DC) 88,990 97,732 Fixed capital investment, total 667,428 732,992 II. Other outlays A. Start up cost (6% of FCI) 40,046 43,980 B. Working capital (5% of FCI) 33,371 36,650 C. Cost of licensing, research and development, {2,3,4} 29,452 29,452

Total capital investment (TCI) 770,297 843,074


The estimated parameters for the calculation of the annual equivalent investment cost are shown in Table 15:

TABLE 15: Estimated parameters for the annual equivalent investment cost

Type N ieff Annual cost factor Absorption refrigeration 25 0.1 11%

To calculate the cost of electricity, the formula in Equation 31 was used (Ólafsson, 2007):

..

(31) where Ec = Electricity cost [USD];

mgeo = Mass flow of hot geothermal water [kg/s]; mctower: = Mass flow of the cooling water for the condenser and absorber equipment [kg/s]; Wp = Work of the pump [kW];

hr = Maximum utilization hours [hour]; Pe = Price of electricity [USD].

For calculating the hot water cost, the formula in Equation 32 was used:

3.6 (32) where HWc = Hot water cost [USD];

Phw = Price of hot geothermal water [USD/(kg °C)]; mhw = Mass flow of hot geothermal water [kg/s]; Tin-hot = Inlet temperature of hot geothermal water to the desorber [°C]; Tout-hot = Outlet temperature of hot geothermal water coming from desorber [°C]; hr = Maximum utilization hours [h].

For calculating the cost of the cold water, the formula in Equation 33 was used:

3.6 (33)

where CWC = Cold water cost [USD]; Pcw = Cold water price [USD/kg]; mctower = Mass flow rate of cooling water going to the condenser, absorber [kg/s].

For calculating the operation and maintenance cost, the formula in Equation 34 was used:

0.005 (34) where OMC = Operation and maintenance cost [USD];

TCI = Total capital investment [USD]; Qe = Evaporator heat [kW];

Hence, the total annual costs can be calculated for the Berlín refrigeration cycle (Table 16). According to Figure 29, the change in the cooling inlet temperature in the condenser and absorber will generate a reduction in the COP (Coefficient of performance of the cycle) and will also have a significant impact on the cost of the equipment, mainly on the rectifier with about a 90% increase.


TABLE 16: Total annual cost results for the Berlín refrigeration cycle

Total cost 0.5 MW power

T=8°C [USD]


Annual capital cost 76,299.00 83,117.00 Electricity cost 6,966 7,956 Hot water cost 22,416 32,403 Cold water cost (1) 29,820 36,447 Cold water cost (2) 405,000 495,000 Operation and maintenance 8,674 9,038 Total with cw 1 144,175 168,961 Total with cw 2 519,355 627,514 Total annual cost 331,765 398,238

FIGURE 29: Berlín refrigeration capital cost with condenser inlet temperature and variation of it (right hand side)

5.6.2 Capital and operating cost estimation for the Ahuachapán geothermal field The same calculation method was applied for the case of Ahuachapán geothermal field, and the results are shown in Tables 17, 18 and 19.

-80%

-60%

-40%

-20%

0%

20%

40%

60%

80%

100%

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

Condenser Solution hx. (thermal

compressor)

Solution hx. (NH3

superheat)

Absorber Desorber Rectifier Evaporator

Cap

ital c

ost p

er e

quip

men

t [U

SD]

Investmentcost (8°C)0.5 MW


Variation %


TABLE 17: Capital costs for the refrigeration cycle for the Ahuachapán field


[USD]


[USD] Condenser 58,122 52,756 Solution hx. (thermal compressor) 21,068 24,454

Solution hx. (NH3 superheated) 7,705 7,435 Absorber 129,465 154,433 Desorber 15,435 41,417 Rectifier 25,701 77,622 Evaporator 44,539 30,246 Pumps - - Thermal compressor pump 1,473 1,473 Evaporator bleed pump 1,473 1,473 Annual cost from investment - annual capital cost 304,981 391,309

TABLE 18: Ahuachapán, cost breakdown for the refrigeration cycle

I. Fixed capital investment (FCI) 0.75 MW power

T=8°C [USD]


A. Direct cost (DC) 1. Onsite cost (ONSC) purchased equipment cost( PEC):

1. Heat exchangers 302,036 388,364 2. Pumps 3,116 3,871 3. Pipes in system (5% of 1 +2) 15,258 19,612 4. Electrical control & monitor system (30% of 1+2+3) 96,123 123,554

Total onsite cost: 416,533 535,401 2. Offsite cost (OFSC) 1. Civil structural and architectural work 20% of ONSC 83,306 107,080

2. Service facilities (hot source & cold sink connection) (25% ONSC) 104,133 133,850

3. Contingencies (15% of ONSC) 62,480 80,310 Total direct cost (DC) 666,452 856,641 B. Indirect cost (IDC) 1. Engineer. and supervis. (15% of DC) 99,968 128,496 2. Construction cost including contractor´s profit (15% of DC) 99,968 128,496

3. Contingencies (20% of DC) 133,290 171,328 Fixed capital icome, total 999,678 1,284,961 II. Other outlays A. Start up cost (6% of FCI) 59,981 77,098 B. Working capital (5% of FCI) 49,984 64,248 C. Cost of licensing, research and development, {2,3,4} 29,452 29,452

Total capital investment (TCI) 1,139,095 1,455,759


TABLE 19: Total annual cost results for Ahuachapán refrigeration cycle

0.75 MW power

T=8°C [USD]


Annual capital cost 110,847 140,512 Electricity cost 6,664 8,235 Hot water cost 38,028 93,507 Cold water cost (1) 29,820 39,760 Cold water cost (2) 405,000 540,000 Operation and maintenance 12,819 14,402 Total with cw 1 198,178 296,416 Total with cw 2 573,358 796,656 Total annual cost 385,768 546,536

Figure 30 shows the increase and decrease of equipment costs when changing the cooling temperature at the entrance of the condenser and absorber.

FIGURE 30: Ahuachapán refrigeration capital cost variation with condenser inlet temperature Table 20 shows the comparison of the costs of installing and operating the ammonia and water absorption refrigeration cooling cycle for the two fields, while Figure 31 shows the same results graphically. It is important to note that the refrigeration load for the Ahuachapán refrigeration cycle is 0.75 MW, which is different from the Berlín geothermal cycle which has a refrigeration load of 0.5 MW. Therefore, a better comparison will result from temperature changes in the cooling water.

TABLE 20: Cost comparison for the refrigeration projects in the Berlín and Ahuachapán fields

T=8°C [USD]

T=16°C [USD]

Berlín – total annual cost 331,765 398,238 Ahuachapán – total annual cost 385,768 546,536

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Absorber Desorber Rectifier Evaporator

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ital c

ost p

er eq

uipm

ent [

USD

]



Variation


5.7 Results

• For the cooling cycle with the Lithium Bromide fluid, the temperature for the cooling fluid at the condenser and absorber could be increased to reasonable values for a cooling tower.

• When evaluating the location of the equipment in the cooling cycle, stationing the evaporator and condenser away from the power station resulted in greater pipe diameters for the gathering system due to the low pressure of the steam and high specific volume.

• Changing the cooling water temperature from 8 to 16°C, led to a reduction in the COP for the Berlín field, from 0.44 to 0.25; and the cost increased about 20% with respect to the total annual cost.

• Changing the cooling water temperature from 8 to 16°C, led to a reduction in the COP for the Ahuachapán field from 0.4 to 0.16; and the cost increased about 41%, with respect to the total annual cost.

6. CONCLUSIONS The study shows that the refrigeration absorption cycle is not adaptable to tropical climates under the report’s considerations in which a cooling tower is assumed to be the supplier of cold fluid to the condenser and absorber equipment, where temperatures need to be as low as possible. However, there is an alternative: a combined cascading scenario of the cooling and refrigerating cycle. Here, the cooling facility could be supplying, not only the refrigeration cycle, but also to other users who might need it for storage of daily products, cured meat, or as was proposed at the beginning of this report, to hydroponic plantations with no traditional products. The cooling cycle also has the flexibility of being placed at a distance from the power station, by transporting the cooled water through a well insulated gathering system. In general, a further study is needed to evaluate the feasibility of installing and operating the cooling load cycle with a Lithium Bromide mixture. From a simple technical point of view, there are acceptable thermodynamic conditions for the continued study.

AKNOWLEDGEMENTS My gratitude goes to Almighty God, for giving me the opportunity for professional growth and for bringing me to this amazing country, Iceland. Thanks to Dr. Ingvar B. Fridleifsson and Mr. Lúdvík S. Georgsson from UNU-GTP for their full support during my stay, and for the enthusiastic work of the staff: Dorthe, Thórhildur and Margret. Thanks to Eng. Jose Antonio Rodriguez from LaGeo SA de CV, who encouraged me to develop the present report, to my supervisors Dr. Halldór Pálsson and Dr.

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T = 8 [°C] T = 16 [°C]To

tal a

nnua

l cos

t [U

SD]

Cooling temperature in condenser CT [°C]

Berlin total annual cost Ahuachapán total annual cost

FIGURE 31: Comparison of the total annual cost vs. cooling temperature for both plants


Gudrún Saevarsdóttir for their unconditional support, and also for the excellent assistance of: Dr. Páll Valdimarsson. Thanks to two persons that have influenced my life in a tremendous way and helped me to go beyond, De-Hsuan Wang Liang, and José Luis Henriquez. My gratitude to the Salvadorians that I met in Iceland and encouraged me during my time there: Mr. José Antonio Rodriguez and his wife Mrs. Minerva, Mr. Manuel Monterrosa, Mr. Francisco Montalvo, Mr. Herbert Mayorga, Mr. Roberto Renderos, Mr. Kevin Padilla, Mr. Manuel Rivera and Mercedes; and finally to my beautiful family and to all the 2007 UNU Fellows for showing me the world.

REFERENCES

ASHRAE, 2002: Handbook of refrigeration. ASHRAE, NY.

Björnsdóttir, U., 2004: Use of geothermal energy for cooling. University of Iceland, Faculty of Engineering, Iceland, MSc thesis, 54 pp.

F-Chart Software, 2004: EES, engineering equation solver. F-Chart Software, website, www.fchart.com/ees/ees.shtml.

FAO, 2005a: FAO/economic and social department/statistics/statistics brief/ major food and agricultural commodities and producers/El Salvador 2005. Food and Agricultural Organization of the United Nations, web page: www.fao.org

FAO, 2005b: FAO/El Salvador/fishery sector/fishery country profile. Food and Agricultural Organization of the United Nations, web page: www.fao.org/fi/fcp/es/SLV/profile.htm

Guerra, C.E., Jacobo, P., and Tenorio, J., 2005: Silica scaling polymerization and inhibition trials at Ahuachapan geothermal field, El Salvador. Proceedings of LaGeo Internal Geothermal Congress 2004, 9 pp.

ITC, 2005: ITC/products and countries/international trade statistics by country and by product group. International Trade Centre, web page: www.intracen.org/tradstat/sitc3-3d/er222.htm.

LaGeo S.A de C.V., 2007: Mass flow production weekly reports. LaGeo S.A. de C.V., Production Department, El Salvador, report.

Ministry of Economy, 2007: El Salvador governmental energy policy. Ministry of Economy, El Salvador, web page: www.minec.gob.sv.

Molina, S.P., Barnett, P., Castro, M., Guerra, E., and Henriquez, J.L., 2005: Silica polymerization and deposition trials at the Berlín geothermal field, El Salvador. Proceedings of the World Geothermal Congress 2005, Antalya, Turkey,CD 4 pp.

Ólafsson, Th., 2007: The use of geothermal heat for refrigeration. University of Iceland, Faculty of Engineering, Iceland, MSc thesis, 108 pp.

Rodríguez, J.A., and Herrera, A., 2007: Phased development at Ahuachapán and Berlín geothermal fields. In: Rodríguez, J.A., Lectures on geothermal in Central America. UNU-GTP, Iceland, report 2, 1-5.

Rogowska. A., and Szaflik. W., 2005: Cooling load production in sorption cycles supplied by a geothermal heat source for air conditioning systems. Proceedings of the World Geothermal Congress 2005, Antalya, Turkey, CD 12 pp.

UT, 2007: UT/Statistics/Resource. Transactions Unit, web page: www.ut.com.sv.

cost estimation of using an absorption refrigeration system

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